System and method for monitoring ablation size

ABSTRACT

A system for monitoring ablation size is provided and includes a power source including a microprocessor for executing at least one control algorithm. A microwave antenna is configured to deliver microwave energy from the power source to tissue to form an ablation zone. An ablation zone control module is in operative communication with memory associated with the power source. The memory includes one or more data look-up tables including one or more electrical parameter associated with the microwave antenna. The one or more electrical parameters corresponding to an ablation zone having a radius. The one or more electrical parameters include a threshold value, wherein when the threshold value is met the power source is adjusted to form an ablation zone of suitable proportion.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patent application Ser. No. 14/064,846 filed Oct. 28, 2013, which is a divisional application of U.S. patent application Ser. No. 12/606,769 filed on Oct. 27, 2009, now U.S. Pat. No. 8,568,401, the entire contents of all of which are incorporated by reference herein.

BACKGROUND

1. Technical Field

The present disclosure relates to systems and methods that may be used in tissue ablation procedures. More particularly, the present disclosure relates to systems and methods for monitoring ablation size during tissue ablation procedures in real-time.

2. Background of Related Art

In the treatment of diseases such as cancer, certain types of cancer cells have been found to denature at elevated temperatures (which are slightly lower than temperatures normally injurious to healthy cells). These types of treatments, known generally as hyperthermia therapy, typically utilize electromagnetic radiation to heat diseased cells to temperatures above 41° C. while maintaining adjacent healthy cells at lower temperatures where irreversible cell destruction will not occur. Procedures utilizing electromagnetic radiation to heat tissue may include ablation of the tissue.

Microwave ablation procedures, e.g., such as those performed for menorrhagia, are typically done to ablate the targeted tissue to denature or kill the tissue. Many procedures and types of devices utilizing electromagnetic radiation therapy are known in the art. Such microwave therapy is typically used in the treatment of tissue and organs such as the prostate, heart, and liver.

One non-invasive procedure generally involves the treatment of tissue (e.g., a tumor) underlying the skin via the use of microwave energy. The microwave energy is able to non-invasively penetrate the skin to reach the underlying tissue. However, this non-invasive procedure may result in the unwanted heating of healthy tissue. Thus, the non-invasive use of microwave energy requires a great deal of control.

Currently, there are several types of systems and methods for monitoring ablation zone size. In certain instances, one or more types of sensors (or other suitable devices) are operably associated with the microwave ablation device. For example, in a microwave ablation device that includes a monopole antenna configuration, an elongated microwave conductor may be in operative communication with a sensor exposed at an end of the microwave conductor. This type of sensor is sometimes surrounded by a dielectric sleeve.

Typically, the foregoing types of sensor(s) are configured to function (e.g., provide feedback to a controller for controlling the power output of a power source) when the microwave ablation device is inactive, i.e., not radiating. That is, the foregoing sensors do not function in real-time. Typically, the power source is powered off (or pulsed off) when the sensors are providing feedback (e.g., tissue temperature) to the controller and/or other device(s) configured to control the power source.

SUMMARY

The present disclosure provides a system for monitoring ablation size in real-time. The system includes a power source including a microprocessor for executing at least one control algorithm. The system includes a microwave antenna configured to deliver microwave energy from the power source to tissue forming an ablation zone. An ablation zone control module is in operative communication with a memory associated with the power source. The memory includes one or more data look-up tables including one or more electrical parameters associated with the microwave antenna. The electrical parameter(s) corresponding to a radius of the ablation zone, wherein the ablation zone control module triggers a signal when a predetermined threshold value of the electrical parameter(s) is measured corresponding to the radius of the ablation zone.

The present disclosure provides a microwave antenna adapted to connect to a power source configured for performing an ablation procedure. The microwave antenna includes a radiating section configured to deliver microwave energy from a power source to tissue to form an ablation zone. An ablation zone control module is in operative communication with a memory associated with the power source. The memory includes one or more data look-up tables including one or more electrical parameters associated with the microwave antenna. The electrical parameter(s) corresponding to a radius of the ablation zone, wherein the ablation zone control module triggers a signal when a predetermined threshold value of the electrical parameter(s) is measured corresponding to the radius of the ablation zone.

The present disclosure also provides a method for indirectly monitoring temperature of tissue undergoing ablation by way of probe impedance. The method includes an initial step of transmitting microwave energy from a power source to a microwave antenna to form a tissue ablation zone. A step of the method includes monitoring complex impedance associated with the microwave antenna as the tissue ablation zone forms. A step of the method includes communicating a control signal to the power source when a predetermined complex impedance is reached at the microwave antenna. Adjusting the amount of microwave energy from the power source to the microwave antenna is another step of the method.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of the present disclosure will become more apparent in light of the following detailed description when taken in conjunction with the accompanying drawings in which:

FIG. 1 is a perspective view of a system for monitoring ablation size according to an embodiment of the present disclosure;

FIG. 2 is a functional block diagram of a power source for use with the system depicted in FIG. 1;

FIG. 3A is a schematic, plan view of the tip of a microwave antenna depicted in FIG. 2A illustrating radial ablation zones having a spherical configuration;

FIG. 3B is a schematic, plan view of the tip of a microwave antenna depicted in FIG. 1 illustrating radial ablation zones having an ellipsoidal configuration;

FIG. 4A-1 is a graphical representation of a real impedance (Zr) versus time (t) curve;

FIG. 4A-2 a graphical representation of a corresponding ablation radii (Ar) versus time (t) curve;

FIG. 4B-1 is a graphical representation of the imaginary impedance (Zi) versus time (t) curve;

FIG. 4B-2 is a graphical representation of corresponding ablation radii (Ar) versus time (t) curve; and

FIG. 5 is a flow chart illustrating a method for monitoring temperature of tissue undergoing ablation in accordance with the present disclosure.

DETAILED DESCRIPTION

Embodiments of the presently disclosed system and method are described in detail with reference to the drawing figures wherein like reference numerals identify similar or identical elements. As used herein and as is traditional, the term “distal” refers to the portion which is furthest from the user and the term “proximal” refers to the portion that is closest to the user. In addition, terms such as “above”, “below”, “forward”, “rearward”, etc. refer to the orientation of the figures or the direction of components and are simply used for convenience of description.

Referring now to FIG. 1, a system for monitoring ablation size is designated 10. The system 10 includes a microwave antenna 100 that is adapted to connect to an electrosurgical power source, e.g., an RF and/or microwave (MW) generator 200 that includes or is in operative communication with one or more controllers 300 and, in some instances, a fluid supply pump 40. Briefly, microwave antenna 100 includes an introducer 116 having an elongated shaft 112 and a radiating or conductive section or tip 114 operably disposed within elongated shaft 112, a cooling assembly 120 having a cooling sheath 121, a handle 118, a cooling fluid supply 122 and a cooling fluid return 124, and an electrosurgical energy connector 126. Connector 126 is configured to connect the microwave antenna 100 to the electrosurgical power source 200, e.g., a generator or source of radio frequency energy and/or microwave energy, and supplies electrosurgical energy to the distal portion of the microwave antenna 100. Conductive tip 114 and elongated shaft 112 are in electrical communication with connector 126 via an internal coaxial cable 126 a that extends from the proximal end of the microwave antenna 100 and includes an inner conductor tip that is operatively coupled to a radiating section 138 operably disposed within the shaft 112 and adjacent the conductive or radiating tip 114 (see FIG. 3A, for example). As is common in the art, internal coaxial cable 126 a includes a dielectric material and an outer conductor surrounding each of the inner conductor tip and dielectric material. A connection hub (not shown) disposed at a proximal end of the microwave antenna 100 operably couples connector 126 to internal coaxial cable 126 a, and cooling fluid supply 122 and a cooling fluid return 124 to a cooling assembly 120. Radiating section 138 by way of conductive tip 114 (or in certain instances without conductive tip 114) is configured to deliver radio frequency energy (in either a bipolar or monopolar mode) or microwave energy (having a frequency of about 500 MHz to about 10 GHz) to a target tissue site. Elongated shaft 112 and conductive tip 114 may be formed of suitable conductive material including, but not limited to copper, gold, silver or other conductive metals having similar conductivity values. Alternatively, elongated shaft 112 and/or conductive tip 114 may be constructed from stainless steel or may be plated with other materials, e.g., other conductive materials, such as gold or silver, to improve certain properties, e.g., to improve conductivity, decrease energy loss, etc. In an embodiment, the conductive tip may be deployable from the elongated shaft 112.

With reference to FIG. 2, a schematic block diagram of the generator 200 is illustrated. The generator 200 includes a controller 300 having one or more modules (e.g., an ablation zone control module 332 (AZCM 332), a power supply 237 and a microwave output stage 238). In this instance, generator 200 is described with respect to the delivery of microwave energy. The power supply 237 provides DC power to the microwave output stage 238 which then converts the DC power into microwave energy and delivers the microwave energy to the radiating section 138 of the microwave antenna 100. The controller 300 may include analog and/or logic circuitry for processing sensed values provided by the AZCM 332 and determining the control signals that are sent to the generator 200 and/or supply pump 40 via a microprocessor 335. The controller 300 (or component operably associated therewith) accepts one or more measured signals indicative of calculated complex impedance associated with the microwave antenna 100 and/or tissue adjacent an ablation zone when the microwave antenna is radiating energy.

One or more modules e.g., AZCM 332, of the controller 300 analyzes the measured signals and determines if a threshold complex impedance has been met. If the threshold complex impedance has been met, then the AZCM 332, a microprocessor 335 and/or the controller instructs the generator 200 to adjust the microwave output stage 238 and/or the power supply 237 accordingly. Additionally, the controller 300 may also signal the supply pump to adjust the amount of cooling fluid to the microwave antenna 100 and/or the surrounding tissue. The controller 200 includes microprocessor 335 having memory 336 which may be volatile type memory (e.g., RAM) and/or non-volitile type memory (e.g., flash media, disk media, etc.). In the illustrated embodiment, the microprocessor 335 is in operative communication with the power supply 237 and/or microwave output stage 238 allowing the microprocessor 335 to control the output of the generator 300 according to either open and/or closed control loop schemes. The microprocessor 335 is capable of executing software instructions for processing data received by the AZCM 332, and for outputting control signals to the generator 300 and/or supply pump 40, accordingly. The software instructions, which are executable by the controller 300, are stored in the memory 336.

One or more control algorithms for predicting tissue ablation size is implemented by the controller 300. More particularly, the concept of correlating complex impedance (e.g., real and imaginary portions of the complex impedance) associated with a particular microwave antenna, e.g., the microwave antenna 100, with an ablation zone “A” having a radius “r” may be used to indicate tissue death or necrosis. More particularly, complex impedance associated with the microwave antenna 100 varies over the course of an ablation cycle due to tissue complex permittivity changes caused by temperature increase (see FIGS. 4A-1 and 4B-1, for example). A relationship of complex impedance as a function of time may be represented by the curves illustrated in FIGS. 4A-1 (real portion of complex impedance) and 4B-1 (imaginary portion of complex impedance). When the microwave antenna 100 has heated tissue to a maximum attainable temperature, an ablation zone “A” having a corresponding radius “r” (e.g., rss) is formed (see FIG. 3A in combination with FIGS. 4A-2 and 4B-2, for example). At this maximum temperature a dielectric constant and conductivity associated with the ablated tissue reach a steady-state condition (this steady-state condition occurs at time tss) that corresponds to a steady-state complex impedance Zss (hereinafter referred to simply as Zss) associated with the microwave antenna 100. That is, because the ablated tissue is in a “near field” of the microwave antenna 100, the ablated tissue essentially becomes part of the microwave antenna 100. Accordingly, when a dielectric constant and conductivity associated with the ablated tissue reaches a steady-state condition, the complex impedance at the microwave antenna 100 also reaches a steady-state condition, e.g., Zss, where Zss includes a real portion Zrss and an imaginary portion Ziss, see FIGS. 4A-1 and 4B-1, respectively.

It should be noted, that Zss may vary for a given microwave antenna. Factors that may contribute to a specific Zss for a given microwave antenna include but are not limited to: dimensions associated with the microwave antenna (e.g., length, width, etc.); type of material used to manufacture the microwave antenna (or portion associated therewith, e.g., a radiating section) such as copper, silver, etc; and the configuration of the radiating section (e.g., dipole, monopole, etc.) and/or a conductive tip (e.g., sharp, blunt, curved, etc) associated with the microwave antenna.

The control algorithm implements one or more model equations and/or curves, e.g., curves depicted in FIGS. 4A-1 and 4B-1, to calculate the Zss associated with the microwave antenna 100 within a specified time range (e.g., t1-tss) not exceeding tss, i.e., time when the ablated tissue is at the steady-state condition (see FIG. 4A-1 or FIG. 4B-1, for example). More particularly, the real and imaginary portions, Zrss and Ziss, respectively, of the Zss of the microwave antenna 100 may be calculated via monitoring and/or measuring of a signal (or pulse) generated by the generator 200. More particularly, a phase (for calculating an imaginary impedance Ziss of the complex impedance) and magnitude (for calculating a real impedance Zrss of the complex impedance) associated with a signal (or pulse) generated by the generator 200 during an ablation procedure may be sampled and monitored. For example, one or more electrical properties (e.g., voltage, current, power, impedance, etc.) associated with a signal (or pulse) generated by the generator 200 may be sampled and monitored. More particularly, electrical properties associated with a forward and reflected portion of the signal generated by the generator 200 is sampled and monitored. For example, in one particular embodiment, forward and reflected power, Pfwd and Pref respectively, of a signal for ablating tissue is measured by the AZCM 332, controller 300, microprocessor 337 or other suitable module associated with the generator 200 and/or controller 200. Thereafter, the power standing wave ratio (Pswr) is calculated using the equation:

$\begin{matrix} {P_{SWR} = \frac{P_{fwd} + P_{ref}}{P_{fwd} - P_{ref}}} & (1) \end{matrix}$

where Pfwd is the power associated with the generated signal (i.e., forward signal) and Pref is the power associated with the reflected signal. Those skilled in the relative art can appreciate that with the Pswr, Pfwd and Pref calculated the real portion of the complex impedance at the steady-state condition, e.g., Zss, of the microwave antenna 100 may be calculated. More particularly, the phase difference between the forward and reflected power may be used to calculate the imaginary portion Ziss of the complex impedance and the magnitude difference between the forward and reflected power may be used to calculate the real portion Zrss of the complex impedance. With Zrss and Ziss known, Zss may be calculated and, subsequently, communicated and/or relayed to one or more modules associated with the controller 300, e.g., AZCM 332, to determine if a predetermined threshold value Zss that corresponds to a desired ablation size has been met. For example, in certain instances, known characteristic impedance associated with connector 126 and/or internal cable 126 a may be employed to determine Zss. More particularly, measurement of Zss may be determined using the equation:

$\begin{matrix} {\frac{Z_{ss} - Z_{o}}{Z_{ss} + Z_{o}} = \frac{P_{SWR} - 1}{P_{SWR} + 1}} & (2) \end{matrix}$

where, Zo is the characteristic impedance associated with the connector 126 and/or internal cable 126 a. The characteristic impedance Zo is an accurate measure of the impedance of the connector 126 and/or internal cable 126 a and takes into account the line losses associated with the connector 126 and/or internal cable 126 a. In this instance, after all the necessary calculations have been carried out, the measurement of Zss will be an accurate representation of the steady-state impedance Zss at the microwave antenna 100 adjacent the ablation zone.

The foregoing algorithms and/or equations are two of many algorithms and/or equations that may be employed to calculate the Zss associated with the microwave antenna 100 such that real-time monitoring of an ablation zone may be achieved. For example, one or more model functions ƒ(t) representative of the model curves illustrated in FIGS. 4A-1 and 4B-1 may be utilized in conjunction with the aforementioned equations (or alone) to obtain additional information relevant to Zss. More particularly, a measurement of a slope of a tangent line at a point along either of the curves (e.g., curve illustrated in FIG. 4A-1) is equal to a derivative (dz/dt) of the curve at that point. The calculation of the derivative at a particular point along the curve(s) may provide additional information, e.g., rate of change of complex impedance with respect to time. This rate of change associated with complex impedance with respect to time may be utilized, for example, to determine the time it takes to go from Z4 to Zss during an ablation procedure.

The microwave antenna 100 of the present disclosure may be configured to create an ablation zone “A” having any suitable configuration, such as, for example, spherical (FIG. 3A), hemispherical, ellipsoidal (FIG. 3B where the ablation zone is designated “A-2”), and so forth. In one particular embodiment, microwave antenna 100 is configured to create an ablation zone “A” that is spherical (FIG. 3A). As noted above, when the microwave antenna 100 has heated tissue in the “near field” to a maximum temperature, a dielectric constant and conductivity associated with the ablated tissue reaches a steady-state that corresponds to a steady-state complex impedance Zss associated with the microwave antenna 100. Correlating the Zss associated with the microwave antenna 100 with the ablated tissue (i.e., ablated tissue, where the dielectric constant and conductivity are in a steady-state condition), indicates a specific size (e.g., radius rss) and shape (e.g., spherical) of the ablation zone “A.” Thus, a measure of Zss associated with the microwave antenna 100 corresponds to an ablation zone “A” having a radius r, e.g., rss. The control algorithm of the present disclosure uses known or calculated steady state complex impedances associated with specific microwave antennas at specific radii to predict an ablation size. That is, complex impedances, e.g., Zss, associated with a specific microwave antenna, e.g., microwave antenna 100, and corresponding radius, e.g., rss, are compiled into one or more look-up tables “D” and are stored in memory, e.g., memory 336, accessible by the microprocessor 335 and/or the AZCM 332. Thus, when the complex impedance for a specific microwave antenna, e.g., microwave antenna 100, reaches Zss one or more modules, e.g. AZCM 332, associated with the controller 300, commands the controller 200 to adjust the power output to the microwave antenna 100 accordingly. This combination of events will provide an ablation zone “A” with a radius approximately equal to rss.

In an embodiment, for a given microwave antenna, e.g., microwave antenna 100, impedance measurements may be taken at times prior to tss, e.g., times t1-t4. In this instance, complex impedances, e.g., Z1-Z4 (for illustrative purposes and clarity, Z1-Z4 are defined by both the real and imaginary portions of the complex impedance), associated with the microwave antenna 100 may be correlated with an ablation zone “A” defined by a plurality of concentric ablation zones having radii r1-r4 (collectively referred to as radii “r”) when measured from the center of the ablation zone “A.” More particularly, the complex impedances Z1-Z4 and corresponding radii “r” may be correlated with each other in a manner as described above with respect to Zss and rss (see FIG. 3A in combination with FIGS. 4A-1 and 4B-1, for example). In this instance, when specific complex impedance, e.g., Z3, is met one or more modules, e.g. AZCM 332, associated with the controller 300, commands the controller 200 to adjust the power output to the microwave antenna 100 accordingly.

AZCM 332 may be a separate module from the microprocessor 335, or AZCM 332 may be included with the microprocessor 335. In an embodiment, the AZCM 332 may be operably disposed on the microwave antenna 100. The AZCM 332 may include control circuitry that receives information from one or more control modules and/or one or more impedance sensors (not shown), and provides the information to the controller 300 and/or microprocessor 335. In this instance, the AZCM 332, microprocessor 335 and/or controller 300 may access look-up table “D” and confirm that a particular complex impedance (e.g., Zss) associated with microwave assembly 100 that corresponds to a specific ablation zone, e.g., specific ablation zone having a radius rss has been met and, subsequently instruct the generator 200 to adjust the amount of microwave energy being delivered to the microwave antenna. In one particular embodiment, look-up table “D” may be stored in a memory storage device (not shown) associated with the microwave antenna 100. More particularly, a look-up table “D” may be stored in a memory storage device operatively associated with handle 118 and/or connector 126 of the microwave antenna 100 and may be downloaded, read and stored into microprocessor 335 and/or memory 336 and, subsequently, accessed and utilized in a manner described above; this would do away with reprogramming the generator 200 and/or controller 300 for a specific microwave antenna. The memory storage device may also be configured to include information pertaining to the microwave antenna 100. For example, information such as, the type of microwave antenna, the type of tissue that the microwave antenna is configured to treat, the type of ablation zone desired, etc. may be stored into the storage device associated with the microwave antenna. In this instance, for example, generator 200 and/or controller 300 of system 10 may be adapted for use with a microwave antenna configured to create an ablation zone, e.g. ablation zone “A-2,” different from that of microwave antenna 100 that is configured to create an ablation zone “A.”

In the embodiment illustrated in FIG. 1, the generator is shown operably coupled to fluid supply pump 40. The supply pump 40 is, in turn, operably coupled to the supply tank 44 (FIG. 2). In embodiments, the microprocessor 335 is in operative communication with the supply pump 40 via one or more suitable types of interfaces, e.g., a port 240 operatively disposed on the generator 200, which allows the microprocessor 335 to control the output of a cooling fluid 42 from the supply pump 40 to the microwave antenna 100 according to either open and/or closed control loop schemes. The controller 300 may signal the supply pump 40 to control the output of cooling fluid 42 from the supply tank 44 to the microwave antenna 100. In this way, cooling fluid 42 is automatically circulated to the microwave antenna 100 and back to the supply pump 40. In certain embodiments, a clinician may manually control the supply pump 40 to cause cooling fluid 42 to be expelled from the microwave antenna 100 into and/or proximate the surrounding tissue.

Operation of system 10 is now described. In the description that follows, it is assumed that the losses associated with the connector 126 and/or cable 162 a of the microwave antenna 100 are negligible and thus, are not needed in calculating and/or determining a complex impedance of the microwave antenna 100 adjacent the ablation zone during the ablation procedure. Alternatively, the losses associated with the connector 126 and/or cable 162 a of the microwave antenna 100 may be calibrated out of measurement (or other suitable methods) and utilized in calculating and/or determining a complex impedance of the microwave antenna 100 adjacent the ablation zone during the ablation procedure. Initially, microwave antenna 100 is connected to generator 200. In one particular embodiment, one or more modules, e.g., AZCM 332, associated with the generator 200 and/or controller 300 reads and/or downloads data from a storage device associated with the antenna 100, e.g., the type of microwave antenna, the type of tissue that is to be treated, etc. Microwave antenna 100 may then be positioned adjacent tissue (FIG. 3A). Thereafter, generator 200 may be activated supplying microwave energy to radiating section 138 of the microwave antenna 100 such that the tissue may be ablated. During tissue ablation, when a predetermined complex impedance, e.g., Zss, at the microwave antenna 100 is reached, the AZCM 332 instructs the generator 200 to adjust the microwave energy accordingly. In the foregoing sequence of events the AZCM 332 functions in real-time controlling the amount of microwave energy to the ablation zone such that a uniform ablation zone of suitable proportion (e.g., ablation zone “A” having a radius rss) is formed with minimal or no damage to adjacent tissue.

With reference to FIG. 5 a method 400 for monitoring temperature of tissue undergoing ablation is illustrated. At step 402, microwave energy from generator 200 is transmitted to a microwave antenna 100 adjacent a tissue ablation site. At step, 404, complex impedance associated with the microwave antenna is monitored. At step 406, a detection signal is communicated to the generator 200 when a predetermined complex impedance Zss is reached at the microwave antenna 100. At step 408, the amount of microwave energy from the generator 200 to the microwave antenna 100 may be adjusted.

From the foregoing and with reference to the various figure drawings, those skilled in the art will appreciate that certain modifications can also be made to the present disclosure without departing from the scope of the same. For example, one or more directional couplers (not shown) may be operatively associated with the generator 200, controller 300 and/or AZCM 332, and configured to sample the forward, reflected, and/or load power portions of an output signal (or pulse) and direct the sampled signal to the AZCM 332. More particularly, the directional coupler provides samples of the forward and reflected signal (or pulse) generated by the generator 200. The power, magnitude and phase of the generated output signal may be obtained or calculated from the measured forward and reflected signals by conventional algorithms that may employ one or both of the aforementioned equations (1) (2), or other suitable equation.

It should be noted that energy values or parameters (e.g., power, voltage, current, impedance, magnitude and phase) of an output pulse are valid at the output of generator 200. That is, the connector 126 and/or internal cable 126 a may include transmission line losses. In order to get a more accurate reading and/or measurement of the energy values or parameters that are delivered to the microwave antenna 100 and/or reflected back to the generator 200, one would have to know the actual transmission line losses associated with connector 126 and/or internal cable 126 a. Accordingly, in some instances, AZCM 332 (or other suitable module or component associated with the controller 300) may be configured to adjust and/or calibrate Zss to compensate for losses associated with connector 126 and/or internal cable 126 a. For example, line loss information associated with the connector 126 and/or internal cable 126 a may be determined and stored into memory 336 and accessed during an ablation procedure by the AZCM 332 and, subsequently, used in determining if a predetermined threshold value of Zss has been met. Thus, in an embodiment, loss information for connector 126 and/or internal cable 126 a may be determined and, subsequently, stored in memory 336 and accessed by one or more modules, such as, for example, a calibration module 600 or other suitable module (e.g., AZCM 332) for later use. The loss information for connector 126 and/or internal cable 126 a may be determined by any suitable device and/or method. For example, the loss information for connector 126 and/or internal cable 126 a may be determined via network analyzer 602. In one particular embodiment, the network analyzer 602 may be an integral part of generator 200 (e.g., part of calibration module 600) or alternatively, the network analyzer 602 may be a separate handheld device that is in operative communication with generator 200. The network analyzer 602 may be used to perform a diagnostic test of connector 126 and/or internal cable 126 a. The network analyzer 602 may function in a fashion similar to most conventional network analyzers that are known in the available art. That is, the network analyzer 602 may determine the properties that are associated with connector 126 and/or internal cable 126 a, and more particularly, those properties that are associated with connector 126 and/or internal cable 126 a that affect the reflection and/or transmission of an output signal, such as, for example, the characteristic impedance Zo of connector 126 and/or internal cable 126 a. In embodiments, the network analyzer 602 may be narrow band or single frequency, e.g., microwave frequency utilized by system 10, which, in turn, may reduce the complexity of the system 10.

As noted above, the control algorithm of the present disclosure implements one or more model equations and/or curves to calculate Zss within the time range t1-tss. In certain instances, however, for a particular probe, system 10 and operative components associated therewith, e.g., AZCM 332, may be configured to monitor ablation zone size after time tss. More particularly, system 10 may be configured to deliver “x” amount of electrosurgical energy to microwave antenna 100 for “n” more seconds such that an ablation zone “A” having a radius “y” is achieved.

While several embodiments of the disclosure have been shown in the drawings and/or discussed herein, it is not intended that the disclosure be limited thereto, as it is intended that the disclosure be as broad in scope as the art will allow and that the specification be read likewise. Therefore, the above description should not be construed as limiting, but merely as exemplifications of particular embodiments. Those skilled in the art will envision other modifications within the scope and spirit of the claims appended hereto. 

1-20. (canceled)
 21. A method for monitoring tissue ablation, the method comprising: transmitting microwave energy from a power source to a microwave antenna to form a tissue ablation zone; measuring complex impedance associated with the microwave antenna as the tissue ablation zone forms; generating a control signal based on a comparison of the measured complex impedance with a threshold value of complex impedance corresponding to at least one desired property of the tissue ablation zone; and adjusting at least one property of the microwave energy transmitted from the power source to the microwave antenna in response to the comparison.
 22. The method according to claim 21, further comprising: measuring forward power and reflected power of the microwave energy transmitted from the power source to the microwave antenna as the tissue ablation zone forms.
 23. The method according to claim 22, further comprising: measuring a phase difference between the forward power and the reflected power generated by the power source to calculate an imaginary portion of the complex impedance associated with the microwave antenna as the tissue ablation zone forms.
 24. The method according to claim 23, further comprising: measuring a magnitude difference between the forward power and the reflected power to calculate a real portion of the complex impedance.
 25. The method according to claim 21, wherein the threshold value of complex impedance is a complex value of impedance in which at least a portion of the microwave antenna and the tissue ablation zone are in a steady-state condition relative to each other.
 26. The method according to claim 21, wherein the at least one desired property of the tissue ablation zone selected from the group consisting of size, shape, radius, and combinations thereof.
 27. The method according to claim 21, further comprising: supplying a cooling fluid to the microwave antenna for cooling at least one of the microwave antenna or the tissue ablation zone.
 28. The method according to claim 21, wherein adjusting at least one property includes adjusting at least one property of the microwave energy selected from the group consisting of voltage, current, power, and impedance.
 29. A method for monitoring tissue ablation, the method comprising: transmitting microwave energy from a power source to a microwave antenna to form a tissue ablation zone; measuring complex impedance associated with the microwave antenna as the tissue ablation zone forms; generating a control signal at a controller disposed within the power source, the control signal being based on a comparison of the measured complex impedance with a threshold value of complex impedance corresponding to a radius of the tissue ablation zone; and adjusting at least one property of the microwave energy transmitted from the power source to the microwave antenna in response to the comparison.
 30. The method according to claim 29, accessing from a memory storage device associated with the microwave antenna by the controller, the memory storage device including the threshold value of complex impedance.
 31. The method according to claim 30, wherein the threshold value of complex impedance is a complex value of impedance in which at least a portion of the microwave antenna and the tissue ablation zone are in a steady-state condition relative to each other.
 32. The method according to claim 29, wherein adjusting at least one property includes adjusting at least one property of the microwave energy selected from the group consisting of voltage, current, power, and impedance. 